Janusz Lewiński

Metal Complexes of Cinchonine as Chiral Building Blocks: A Strategy for the Construction of Nanotubular Architectures and Helical Coordination Polymers

The first chiral bipyridyl-type metalloligands based on aluminum derivatives of cinchonine (CN-H) were synthesized and characterized by single-crystal X-ray diffraction studies. These bischelate complexes, (CN)(2)AlX [X = Cl (1a), Me (1b)] were found to be effective building blocks for the preparation of novel helical nanotubular architectures as well as chiral bimetallic coordination polymers, as demonstrated by the rational synthesis of a helical structure formed by 1a and ZnCl(2). The applied methodology stands as an exemplary strategy for the rational synthesis of chiral metal-organic frameworks through self-organization driven by nonbonding interactions or coordination, which could potentially find applications in enantioselective separations and catalysis.

Reactivity of Various Four‐Coordinate Aluminum Alkyls towards Dioxygen: Evidence for Spatial Requirements in the Insertion of an Oxygen Molecule into the Al−C Bond

The interaction of dioxygen with various tetrahedral aluminum alkyls, (tBu)3Al.OEt2 (1), tBu2Al(mu-OtBu)2AltBu2 (6), (tBu)2Al(mesal) (2) [mesal=methyl salicylate anion], R2Al(mu-pz)2AlR2 [pz=deprotonated pyrazole, R= Me (3a), Et (3b), and tBu (3c)], R2Al(mu-3,5-Me2pz)2AIR2[3,5-Me2pz = deprotonated 3,5-dimethylpyrazole, R= Me (4a), and Et (4b)], and Et2B(mu-pz)2AlEt2 (5), has been investigated. We were particularly interested in the effect of steric hindrances both caused by the metal-bonded substituents and those that result from the nature of the bifunctional ligand used in the oxygenation reaction. In the reaction of 1 with O2, only the formation of the monoalkoxide compound6 was observed. The latter di-tert-butyl compound as well as all planar aluminapyrazoles, that is, the tert-butyl derivative 3c and lower alkylaluminum derivatives with the more demanding 3,5-dimethylpyrazoyl ligands 4a and 4b, are stable under an atmosphere of dry oxygen and ambient conditions. Inspection of the space-filling representation of these compounds has undoubtedly shown that the bulky tert-butyl groups or pyrazolyles ligands, respectively, provide steric protection for the metal center from the dioxygen attack. In contrast, the dialkylaluminum derivatives of pyrazole, 3a and 3b, and the diethylaluminum bis(1-pyrazolyl)borate complex 5, all with the metal center eclipsed with respect to the plane defined by the four nitrogen atoms, react smoothly with O2 to form the alkyl(alkoxy)aluminum complexes. In the reaction of 5 with O2 for example, the Et-B bonds remained intact, and the dimeric five-coordinate compound [Et2B(mu-pz)2 Al(mu-OEt)Et]2 (9) was isolated in good yield. The interaction of mononuclear di-tert-butyl chelate complex 2 with O2 at -15 degrees C gives (tBuOO)(tBuO)Al(mu-OtBu)2Al(mesal)2 (7) in high yield, and the presence of the alkylperoxo moiety is a particularly significant point in the resulting product. All the compounds have been characterized spectroscopically, and the structures of 3c, 4a, 6, 7, and 9 have been confirmed by X-ray crystallography. Structural features of 1-6 are discussed and are considered in relation to the possible approach pathways of the O2 molecule to the four-coordinate metal center. This analysis and the observed apparent dissimilarity in the reactions of model four-coordinate aluminum alkyls with O2 clearly show that the stereoelectronic prerequisites are responsible for the fundamentally different reactivity.

Oxygenation of a Me2Zn/α‐Diimine System: A Unique Zinc Methylperoxide Cluster and Evidence for Its Sequential Decomposition Pathways

Interest in the reaction of alkylzinc complexes with O2 has persisted for over 150 years since the pioneering studies by Frankland. The nature of the products, however, has been the subject of intense controversy. The widely accepted freeradical chain-reaction mechanism for these reactions, as found in the vast majority of text books, assumes the initiation by adventitious alkyl radicals (RC) followed by a cascade of fast reactions with little opportunity for the detection of intermediates. Recently, however, we provided unambiguous proof that R2Zn compounds, or their adducts with Lewis bases, have a marked tendency to undergo oxidation of only one alkyl group under controlled conditions with subsequent formation of RZnOOR or RZnOR species, and simultaneously structurally characterized the first examples of zinc alkylperoxides. We also proposed a plausible hypothesis for the mechanism of the reaction of alkylzinc complexes with O2. [5,6] In recent years there has been an increased interest in various radical additions initiated by the R2Zn/O2 system, especially regarding organic substrates which contain donor sites capable of forming the Lewis acid/base adducts with R2Zn that are actually involved in the reaction with O2. [7] This latter fact is usually ignored, and another assumption made in this field, which seems irrefutable, involves an alkyl radical RC (generated through the oxygenation reaction) acting as the chain carrier. Moreover, the most effective initiation systems involve Me2Zn, [7f,g] which, according to recent findings, can be selectively transformed into MeZnOMe without the generation of free MeC radicals. In light of this fact, as well as the lack of structurally characterized ZnOOMe species, it seemed reasonable to wonder how the oxygenated products participate in radical reactions. To gain a more in-depth view of both the role of the supporting ligands and the character of the radical species formed in the reactions of Me2Zn with O2, we have turned our attention to a-diimines, which have been widely used in fundamental coordination chemistry as noninnocent ligands. In the field of zinc chemistry, van Koten et al. have extensively studied the reactions of R2Zn compounds with 1,4-diazabutadiene (R-DAB) ligands and have demonstrated convincingly that this reaction system smoothly forms both paramagnetic and diamagnetic species. Herein we report the synthesis and structural characterization of a novel zinc oxo(methylperoxide) cubane along with the MeOC radical entrapped product, the formation of which involves ZnO OMe bond homolysis. Previous studies have demonstrated that the treatment of Me2Zn with tBu-DAB in diethyl ether at ambient temperature results in the formation of [Me2Zn(tBu-DAB)] (1). [11] According to the authors, the four-coordinate adduct 1 is rather stable under these reaction conditions and only upon heating above 35 8C does it undergo an inner-sphere single electron transfer to give the radical pair [MeZn(tBu-DABC)(MeC)], which subsequently dimerizes to the C C coupled dinuclear compound [{MeZn(tBu-DABC)}2] (2 ; Scheme 1). In light of these findings, we decided to modify the reaction system slightly and, in the first instance, stirred equimolar amounts of tBu-DAB andMe2Zn in toluene at 22 8C for 7 h. [12]

Towards a New Family of Photoluminescent Organozinc 8- Hydroxyquinolinates with a High Propensity to Form Noncovalent Porous Materials

We report on investigations of reactions of tBu(2)Zn with 8-hydroxyquinoline (q-H) and the influence of water on the composition and structure of the final product. A new synthetic approach to photoluminescent zinc complexes with quinolinate ligands was developed that allowed the isolation of a series of structurally diverse and novel alkylzinc 8-hydroxyquinolate complexes: the trinuclear alkylzinc aggregate [tBuZn(q)](3) (1(3)), the pentanuclear oxo cluster [(tBu)(3)Zn(5)(μ(4) -O)(q)(5)] (2), and the tetranuclear hydroxo cluster [Zn(q)(2)](2)[tBuZn(OH)](2) (3). All compounds were characterized in solution by (1)H NMR, IR, UV/Vis, and photoluminescence (PL) spectroscopy, and in the solid state by X-ray diffraction, TGA, and PL studies. Density functional theory calculations were also carried out for these new Zn(II) complexes to rationalize their luminescence behavior. A detailed analysis of the supramolecular structures of 2 and 3 shows that the unique shape of the corresponding single molecules leads to the formation of extended 3D networks with 1D open channels. Varying the stoichiometry, shape, and supramolecular structure of the resulting complexes leads to changes in their spectroscopic properties. The close-packed crystal structure of 1(3) shows a redshifted emission maximum in comparison to the porous crystal structure of 2 and the THF-solvated structure of 3.

Cinchona Alkaloid-Metal Complexes: Noncovalent Porous Materials with Unique Gas Separation Properties**

The most common andeffective approach to design and prepare metal–organicframeworks (MOFs) or porous coordination polymers(PCPs) of desired topology and functionality is based oncoordination-driven self-assembly, and both the correctchoice of metal centers and the engineering of the ligandsfeatures, such as size, flexibility, and directionality of bindingcenters, play a decisive role.

Permanent Porosity Derived From the Self-Assembly of Highly Luminescent Molecular Zinc Carbonate Nanoclusters†

Over the past two decades, crystalline microporous materials have attracted major interest, owing to their applications in gas sorption, separation, catalysis, and sensing. Microporous crystalline materials can be constructed from coordinatively or covalently linked building blocks where rigid or semi-rigid molecular scaffolds separate void spaces of different size and geometry. Prominent examples of porous materials showing polymeric structures include zeolites, hybrid metal–organic frameworks (MOFs) or porous coordination polymers (PCPs), covalent organic frameworks (COFs), and Hbonded supramolecular organic frameworks (SOFs). The formation of bonding interactions between building units is an important factor that defines and controls the stability and robustness of these porous polymeric materials. However, discrete molecules may also pack in the solid state to form 3D assemblies that exhibit high permanent porosity. The resulting noncovalent porous materials (NPMs) are distinct from the above polymeric systems, as they are held together by weak noncovalent crystal-packing forces. Modifications of structure by cocrystallization of different building blocks can tune the microcavities in the solid state, and the material can thus conform to the shape or functionality of guest molecules. Moreover, these materials can be highly solubile, an important advantage in their processing to form porous thin films. NPMs can exhibit extrinsic and/or intrinsic porosity. Intrinsic porosity is associated with the structure of the single-molecule-containing voids, clefts, or cavities, as has been demonstrated for calixarenes, cucurbiturils, cyclodextrins, organic cage compounds, or discrete small organic molecules. In contrast, materials with extrinsic porosity are those where individual molecules pack in the solid-state to form structures with empty spaces between the individual molecules. As discrete molecules tend to form close-packed solids with minimal void volume, extrinsic porosity in NPMs remains a rare phenomenon. The rational design and preparation of NPMs showing extrinsic porosity based on molecular metal complexes is highly challenging and few examples have been reported. We have demonstrated previously that alkylzinc hydroxides RZnOH can be efficiently transformed into multinuclear alkyzinc carbonate nanoclusters or nanomaterials, such as discrete nanoparticulate zinc carbonate aerogels and ZnO nanoparticles. As shown in Scheme 1a, the reactivity of the RZnOH species can be rationalized in terms of the presence of both a proton-reactive Zn C bond and CO2-reactive Zn OH groups. We argued that introduction of an additional auxiliary ligand L to the RZnOH system, followed by CO2 fixation, could lead to the formation of novel molecular building blocks for new extrinsic NPMs. Herein, we report such a strategy in which the construction of a nanosized cluster [Zn10(m6-CO3)4(L)12] (WUT-1; WUT=Warsaw University of Technology) is achieved by fixation of CO2 by the tetranuclear hydroxo precursor [Zn4(m3-OH)2(L)4(tBu)2] (1;

Transformation of ethylzinc species to zinc acetate mediated by O2 activation: reactive oxygen-centered radicals under control.

To date, the formation of alkyl perox-ide (path a, Scheme 1), alkoxide (path b), and oxo spe-cies (path c) in the oxygenation of Zn R species has beenwell-documented. To our knowledge, homolytic MO ORbond cleavage to give the corresponding metal carboxylatespecies (path d, scheme 1) has not been considered as a modeof decomposition of both zinc and other metal alkylperoxides.

Oxozinc carboxylate complexes: a new synthetic approach and the carboxylate ligand effect on the noncovalent-interactions-driven self-assembly.

An atom-efficient and mild synthesis of a series of oxozinc carboxylates [Zn(4)(μ(4)-O)(O(2)CR)(6)] [where R = Ph (2a), p-PhC(6)H(4) (2b), p-MeC(6)H(4) (2c), and p-MeSC(6)H(4) (2d)] from well-defined alkylzinc precursors and H(2)O is described. The molecular and crystal structures of the resulting complexes have been determined by single-crystal X-ray diffraction. A closer examination of their crystal structure provides a direct picture of the effect of the nature of substituents on the molecular self-assembly of the octahedral oxozinc through noncovalent interactions. It was revealed that these discrete oxozinc clusters can form diverse types of noncovalent assemblies ranging from structures representing zeolitic topologies in the case of 2a to soft porous materials with gated voids or open channels for the remaining molecular clusters.

Mechanosynthesis of the hybrid perovskite CH3NH3PbI3: characterization and the corresponding solar cell efficiency

We present a facile mechanochemical route for the preparation of hybrid CH3NH3PbI3 (MAPbI3) perovskite particles with the size of several hundred nanometers for high-efficiency thin-film photovoltaic devices. Powder X-ray diffraction measurements demonstrate that mechanosynthesis is a suitable strategy to produce a highly crystalline CH3NH3PbI3 material showing no detectable amounts of the starting CH3NH3I and PbI2 reagents. Thermal stability measurements based on the thermogravimetric analysis data of mechanosynthesized perovskite particles indicated that the as-ground MAPbI3 is stable up to 300 °C with no detectable material loss at lower temperatures. The optical properties of newly synthesized perovskite particles were characterized by applying steady state absorption and fluorescence spectroscopy, which confirmed a direct band-gap of 1.48 eV. Time resolved single photon counting measurements revealed that 70% of charges undergo recombination with a 61 ns lifetime. The solar cell devices made from mechanosynthesized perovskite particles achieved a power conversion efficiency of 9.1% when applying a one step deposition method.

Solid-state conversion of the solvated dimer [{tBuZn(μ-OtBu)(thf)}2] into a long overlooked trimeric [{tBuZnOtBu}3] species.

Interest in zinc alkoxides originated with the pioneering studies of Frankland. Alkylzinc alkoxides have attracted much attention because of their rich structural diversity and potential practical applications as versatile single-source precursors for the preparation of nanosized ZnO particles. Detailed structural information concerning the extent of coordination and association in these species is important in understanding their physical behavior and chemical reactivity. For unsolvated RZnOR species, dimeric (I), cyclic (IIa) or roof-like trimeric (IIb) (Scheme 1), and tetrameric cubane structures have been considered. 4] Until now, only two basic structural motifs have been crystallographically confirmed, the dimer [{RZn(m-OR’)}2] [3d, 5] and tetrameric cubane [{RZn(m3-OR’)}4]. [3b,c,d,e, 6] In some instances, ligand redistribution takes place, and the formation of trinuclear [(RZn)2Zn(OR’)4] aggregates with a linear chain arrangement [2, 3a,5, 7] or double-cubes [(RZn)6Zn(OR)8] has been observed. [6d, 8] Crucially, no examples of structurally characterized trimeric [{RZnOR}3] forms have been previously reported, although the related well-characterized heavier sulfur and selenium analogues with six-membered Zn3S3 or Zn3Se3 rings are known. 9] Already almost two decades ago, Power and co-workers wrote: “It is the present rarity of its occurrence, in comparison to the dimer or tetramer, that is the most striking feature. [...] Conclusive proof of the existence of alkylzinc alkoxide or aryloxide trimers still awaits a structural determination of such a compound”. Furthermore, the character of RZnOR species in noncoordinating solvents is a matter for conjecture, and studies on the aggregation state of [tBuZnOtBu] (1) are a classic example of inconsistency in the reported data: cryoscopic molecular weight studies by Coates and Roberts indicated that 1 exists as a mixture of dimers and trimers in benzene, whereas Noltes and Boersma concluded that only a trimer exists. In the course of our studies on the structure and reactivity of [ZntBu2], [13, 14] we isolated and structurally characterized the solvated dimeric alkoxide [{tBuZn(m-OtBu)(thf)}2] (1·thf). Herein, we report on a solid-state transformation of 1·thf under mild conditions to give trimer [{tBuZnOtBu}3] (13) with an overlooked unique cluster structure IIc, which upon grinding is converted into tetrameric cubane [{tBuZnOtBu}4] (14) (Scheme 2). In addition, we examine the